1. Field of the Invention
The present invention relates to a multi-ply molding hot-runner mold for molding an injection molded product of synthetic resin material which has a cross-sectional structure comprising a plurality of layers.
2. Description of the Related Art
A stretch-blow molded bottle is required to have barrier properties against oxygen and carbon dioxide, heat resistance and other properties. To meet these requirements, a three-layered preform structure is formed by injection molding. The preform is blow molded into the bottle. The three-layered structure of the preform has inner and outer layers of structural resin material such as polyethylene terephthalate with an intermediate layer interposed therebetween and made of another resin material. Many hot-runner molds used for injection molding such a multi-ply product are known as from Japanese Utility Model Publication No. Sho 63-37222, Japanese Patent Laid-Open No. Sho 63-107525, Japanese Patent Laid-Open No. Sho 63-221024, Japanese Patent Laid-Open No. Sho 63-239022 and so on.
When a product having a multi-ply cross section is to be injection molded from a combination of different resin materials, a difficulty is encountered in that these resin materials have different molding temperatures. Particularly, polyethylene terephthalate (hereinafter called "PET" has a molding temperature higher by about 50 degrees C. than that of ethylene-vinyl alcohol copolymer (hereinafter called "EV-OH"), an effective barrier material. At temperatures higher than the molding temperature of EV-OH, it degrades or pyrolytically decomposes thereby interrupting the molding process. It is thus required to control the temperatures of the respective runners conducting the different resin materials to corresponding injection ports.
Where a single hot-runner mold includes two hot-runners for conducting two different resin materials as described in Japanese Utility Model Publication No. Sho 63-37222, it is impossible to control temperatures of the respective hot-runners at the respective resin molding temperatures which differ from each other by over 50 degrees C.
On the other hand, Japanese Patent Laid-Open Nos. Sho 63-107525 and 63-239022 disclose hot-runner molds which comprise different pieces for the runners of the respective resin materials. The molds have spacer blocks interposed and stacked between the mold pieces along interface areas. Although the material of the spacer blocks is not understood from these Japanese Patent Laid-Opens, it is reasonable to believe that these blocks are made of metal since they must form runners between the upper and lower blocks. With respect to thermal properties, such structures will not be different from hot-runner molds comprising two runners for the different resin materials. Therefore, even if the hot-runner mold pieces are independently controlled in temperature, a heat exchange will occur between the spacer blocks due to the inter-solid heat transfer. It is thus impossible for the molding temperatures of respective resin materials to be accurately controlled in the respective spacer blocks. It is believed that the separate mold pieces of the prior art permit forming runners by separate grooving of the pieces. However, the prior art does not disclose any mold structure which permits easy control of the different molding temperatures of different resin materials.